• No results found

Basalt flows identified within the drill cores in the Auckland pit reveal that there are three individual lavas flows that may be either pahoehoe, aa, and/or transitional lava flows.

Pahoehoe lavas, are produced by inflation of the lava flow beneath a chilled surface crust (Hon et al., 1994), and are the most abundant lava type in the Auckland pit logged in drill core. All pahoehoe flow units observed contain planar upper and lower contacts with very small spherical vesicles preserved just within these margins. The vesicles increase in size from the margins towards the centre of the lava flow. The paheohoe lavas identified in this study, show an increase in the vesicle size from both the upper and lower zones towards the centre, however, the concentration of vesicles decrease towards the centre; the central part of each flow is mainly dense, lacking vesicles (Figs. 3.9 to 3.11).

Pahoehoe lavas propagate by building out from small toes at the flow front beneath chilled, smooth, low-relief surface crusts (Hare & Cas, 2005). Pahoehoe sheet flows are produced by the radial spreading of pahoehoe toes on shallow slopes and coalescence of the surface crusts of adjacent toes, forming a continuous sheet under which molten lava is fed (Hon et al., 1994). Steady inward thickening of the upper and lower crusts occurs around a constant fluid layer of freshly supplied lava (Hare & Cas, 2005). Some gas bubbles in the molten interior lava are trapped in the chilled flow base forming the lower vesicular zone, but most rise buoyantly to the top of the lava, where they are initially trapped beneath, and with further cooling, are incorporated within the upper crust, forming the upper vesicular zone; a dense core represents the non-vesicular zone.

The gas bubbles in lavas are not generally subject to shear stresses, hence vesicles are usually discrete and spherical, and locally the distribution is homogeneous (Aubele et al., 1988; Wilmoth & Walker, 1993; Self et al., 1996).

Horizontal vesicle sequences (e.g. vesicle trails) are an exception, as these are interpreted to represent the final stages of lava movement in pahoehoe flows

81

These periodically rise and spread out at the lower parts of the upper vesicular zone.

The Auckland pit lavas are dominated by pahoehoe lava flows, however, the vesicular distribution within some basalt intervals in drill core make it difficult to identify them as a pahoehoe lava flow (C306, C308; Fig. 3.11). Pahoehoe lavas characteristically form only on gentle slopes of less than 2o (Hon et al., 1994;

Cashman & Kauahikaua, 1997). Although the topography prior to volcanism was more irregular and possibly slightly steeper, it must have been relatively flat to allow inflation of the pahoehoe flows to occur (Hare & Cas, 2005).

Aa lavas are generally emplaced at higher viscosities than pahoehoe lavas, and autobrecciation occurs during emplacement (Macdonald, 1953; Lipman &

Banks, 1987). Talus from cooled, broken surface crust is dumped over the flow front and the side margins, and falls to the ground surface, or is incorporated into the fluid flow (Crisp & Baloga, 1994). The talus is overridden by lava upwelling from the fluid inner core, therefore producing a fragmented basal layer overlain by a coherent lava flow. The lava undergoes autobrecciation because the viscosity or yield strength of the lava beneath the crust is too high for it to flow within and heal torn-apart portions (Rowland & Walker, 1990). The exteriors of the autobrecciated clasts are characteristically rough as the ripped viscous lava retains an irregular shape.

Gas bubbles do not migrate significantly during emplacement due to the high viscosity of the lava. Vesicles in aa lava flows are generally irregular shapes, interconnected to a high degree and their distribution is heterogeneous (Polacci &

Papale, 1997; Cashman et al., 1999; Polacci et al., 1999; Hare & Cas, 2005), because the lava is more viscous and less able to homogenise. The lack of segregation veins in aa flows is probably because the lavas are to viscous for residual melt to migrate through the surrounding crystal mush, as it does in pahoehoe flows.

Broken core represented by facies C observed within the basalt flows of the Auckland pit, was probably generated on steeper terrain within the Auckland

pit area, where the talus from the broken surface crust was thrown over the flow front and then incorporated into the interior of the lava flow.

Transitional lavas contain features which are common to both pahoehoe and aa lava flows (Hare & Cas, 2005). The flows observed in drill core commonly contain rubbly material at the top and base surrounding a coherent interior that can be subdivided into a upper vesicular zone, non-vesicular zone and lower vesicular zone (e.g. Log B19, top, 24.5 – 25 m, and base, 36.5 – 38.5 m; Fig.

3.10). The weathered vesicular clasts of scoriaceous basalt have been entrained into the interior of the flows during emplacement (e.g. Log C306, 3.2 – 7 m; Fig 3.9). Transitional lavas are generally considered to be either aa lava flows or pahoehoe flows, autobrecciated behind the flow-front (Keszthelyi, 2002).

Possibly the magma was significantly undercooled prior to eruption, as evidenced by higher crystallinity and/or the eruption rate was initially higher than the other lava types. It is not clear which (if either) mechanism developed the transitional lava type in this study.

Facies A.1 is moderately vesicular basalt that occurs at the top and base of lava flow intervals. Facies A.1 is likely to be part of a pahoehoe lava flow which usually has an upper vesicular and lower vesicular zone; furthermore, the vesicle size tends to increase towards the centre of the basalt flow within this facies.

Facies A.2 is basalt lava with horizontal vesicle trails up to 10 – 40 mm apart. This facies represents the final stages of lava movement in a pahoehoe lava flow, as volatiles are concentrated in the residual melt during crystallisation and periodically rise and spread out at the lower parts of the upper vesicular zone.

Facies A.3 is high strength, dense, coherent, non-vesicular basalt produced predominantly within a pahoehoe flow. Vesicles which migrate and are trapped within the lower and upper vesicular zones leaving a dense coherent centre; this vertical facies arrangement is commonly associated with pahoehoe lava flows.

Facies A.4 is a poorly vesicular basalt that occurs in-between the non-vesicular and upper and lower non-vesicular zones. However, in this study facies A.4

83

represent fluctuations between the dense non-vesicular zone and vesicular zones on the outer parts within the lava flow.

Facies C is highly weathered, broken and moderately vesicular scoriaceous basalt that is likely to be the product of aa and/or transitional lava flow. Facies C is the autobrecciated part of aa/transitional lava flow. In some drill cores (e.g. Log C308, 16.5 – 18.30 m; Fig. 3.11) the autobreccia clasts are within a coherent pahoehoe basalt lava flow, indicating that the broken surface crust from the talus was thrown over the flow front moving lava. The broken surface crust was then incorporated into the centre of the lava flow which has then been preserved within the pahoehoe flow as it was deposited and cooled, suggesting a transitional lava flow deposition. In other drill cores (e.g. Log C301, 19.5 – 20.80 m; Fig. 3.10), the breccia is visible at the base of the lava flow indicating, that the breccia has broken apart and was dumped over the flow front, then was overridden by lava from the inner core, therefore producing a fragmented basal layer overlain by a coherent lava flow, typical of an aa lava flow.